1. Field of the Invention
This invention relates to ceramic-coated inorganic granules which exhibit improved adhesion to bituminous surfaces and which also exhibit improved alkalinity characteristics.
2. Description of Related Art
Roofing granules, both natural and artificially color-coated, find extremely wide use in roofing and siding materials. Important applications are in granular surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially embedded in one surface of asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and decorative exterior surface. "Weather-resistant" as used herein includes ultraviolet (UV) radiation-resistant.
Skadulis (U.S. Pat. No. 3,528,842) describes roofing granules which contain an algicide, such as cuprous oxide. As explained by Skadulis, methods have long been known to artificially color roofing granules. This consists primarily of utilizing crushed and screened minerals, in which the granules are coated with a suitable pigment contained in an inorganic matrix or bond. The bond is usually applied in the form of a soluble silicate solution and is insolubized either by heat treatment or a combination of heat treatment and chemical action. This yields a substantially water-insoluble coating which is strongly adhered to the base granule. In carrying out these methods, the pigment is typically uniformly applied to the granular surface with the soluble silicate solution. The soluble silicate solution is converted to an insoluble state by heat treatment and chemical action, as by the addition of clay or an acidic material. This can occur either before, during or after the application of the soluble silicate and pigment mixture onto the mineral granules, in order to obtain a reaction between the clay or acidic material and the alkaline portion of the silicate.
U.S. Pat. No. 3,507,676 (McMahon) describes zinc-compound containing algicidal surfacing granules. The roofing granules contain either zinc metal, zinc oxide (ZnO) or zinc sulfide (ZnS), rather than copper-containing compounds patented by Skadulis. McMahon teaches the use of a pigment grade ZnO in either a first or second coating, with concentrations of about 10 grams ("gms") ZnO per kilogram ("kg") of inorganic roofing granules. McMahon noted that ZnO had been previously used in pigmented granules as an insolubilization agent or pigment, but was utilized in quantities insufficient to be useful for long-term algae-resistance.
Lodge et al. (U.S. Pat. No. 3,255,031) describes the use of a small amount of a borate, such as "borax" and the like, in clay-silicate coating compositions. The addition of about 2 gms borate per kg of inorganic roofing granules apparently allows the temperature of the firing kiln to be reduced below about 450.degree. C., and results in a roofing granule which does not need to be treated with pickling agents to insolubilize the coating. Color benefits such as "lightening" of the color of all pigmented granules and "whitening" of dark-pigmented granules are derived from the use of borate-clay-silicate coatings. Lodge et al. further disclose that borates can be used in a single coating process or in the inner or outer coating of a two layer coating system. Skadulis and McMahon also disclose that borates may be used in the processes of making ceramic-coated roofing granules.
In the preparation of ceramic-coated roofing granules it is important that the resulting granule exhibit a "low alkalinity" (defined by a test described herein). A low alkalinity is indicative of the thoroughness of insolubilization of the ceramic coating. It is also important that the resulting granules adhere to bituminous surfaces, such as asphalt, with a limited degree of adhesive failure as opposed to cohesive failure of the asphalt. It would thus be advantageous if ceramic-coated roofing granules could be made to have improved adhesion while exhibiting an alkalinity which is initially low and remains low for an extended period of time. The present invention is drawn to such ceramic-coated inorganic granules, methods of making such granules, and a composite sheet body for roofing and siding which includes such granules.
Other patents which are evident of the state of the art include U.S. Pat Nos. 1,720,708; 2,070,359; 2,091,991; 2,111,131; 2,225,867; 2,981,636; 3,046,188; 3,484,267; 3,494,727; 3,507,676; 3,598,627; 3,752,696; 3,826,825; 3,884,706; 3,888,176; 3,888,682; 3,888,683; 3,888,684; 3,894,877; 3,985,540; 3,998,644; 4,088,810; 4,092,441; 4,359,505; and 4,378,408.
In particular, U.S. Pat. No. 2,111,131 discloses that small but highly effective amounts of zinc oxide may be dissolved in sodium silicate by effecting the dissolution at elevated temperatures and pressures, such as by introducing steam into an autoclave containing zinc oxide and the silicate. U.S. Pat. No. 2,225,867 discloses that the effectiveness of copper oxides (especially cuprous oxide) as an algicide is increased by the presence of zinc oxide. Furthermore, the grade of zinc oxide (i.e. purity, source, and the like) was not a factor in the efficacy of inhibiting algae growth.
Minnesota Mining and Manufacturing Company, St. Paul, Minn. ("3M") has commercialized since 1990 pigmented roofing granules known under the trade designations "LR-7000" and "LR-7070". These granules have three insolubilized silicate-clay coatings, the first two coatings each having about 35 gms cuprous oxide per kg substrate inorganic granules, with the third coating having no cuprous oxide but alternatively containing pigments to render desired color. None of the three coatings contain zinc oxide or borate.
Assignee's copending application Ser. No. 07/945,127, filed Sep. 15, 1992, describes algae-resistant granules similar to the afore-mentioned LR-7000 and LR-7070 granules. However, improvements have been made wherein color has been incorporated rendering indistinguishable algae-resistant granules to the color of corresponding non-algae-resistant granules. The granules of the latter application include zinc oxide made by the so-called French process and a borate compound in the third layer of a three layer ceramic coating.
Zinc oxide made by the so-called French process and the so-called American process are discussed in detail in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 24, 3rd Ed. John Wiley & Sons (New York) pp. 854-863.
The roofing granule industry continually strives for roofing granules exhibiting improved adhesion and alkalinity characteristics. The present invention is drawn toward the use of zinc oxide made by the French process and a borate compound in a ceramic coating on roofing granules to improve the alkalinity and adhesion characteristics of the coating and resulting granule.